Operation of aerobic bioreactors is highly sensitive to the energy required to treat biodegradable waste, and the time within which treatment is to be completed. Stated differently, given enough time and no reasonable constraint on the expenditure of energy, the challenge to operate a waste treatment system would be de minimis. Since the challenge is to provide a system which economically satisfies the time-energy sensitivity of its operation, the emphasis in the art is to provide the most efficient means for transferring as much oxygen into the bioreactor's reaction mass (aqueous suspension of biomass, namely, of organic solids and microorganisms) as is possible, within the least amount of time, using the least amount of energy, to produce treated water of acceptable quality.
More specifically, this invention relates to a unique combination of an aerobic bioreactor in which biodegradable waste generated by humans and/or other animals, and/or industrial process wastes are aerated with an activated sludge (waste-degrading microorganisms), and a membrane filtration unit which separates acceptably pure water from a recycle stream from the bioreactor, more effectively and economically than any known system. It is known that a membrane filtration device, whether a microfiltration or an ultrafiltration membrane, not only avoids the time penalty of gravity settling technology but also provides a highly effective purification means. What was not appreciated is that the permeate is typically less than 5% by volume of the feedstream flowed over the membranes so that the kinetic energy remaining in the concentrate is substantial. It is this remaining kinetic energy which I have capitalized upon.
The rate of transfer of oxygen limits the biomass concentration in an activated sludge wastewater treatment system (see Aerobic biological Treatment of Wastewaters: Principles and Practice by A. W. Bush Pg. 285-312 Oligodynamics Press 1971). There are numerous references teaching how to aerate a bioreactor (hereafter "reactor"); and, membrane devices have long been known to be highly efficient separating means to filter solids-free permeate ("permeate" for brevity) from the solids-containing concentrate ("concentrate"). But aerating a reactor efficiently is not simply a matter of blowing copious amounts of air through the suspension of solids in the reactor. The oxygen must be transferred to the suspension of solids. How effectively this is done is a measure of the economic success of the reactor.
Mindful of the foregoing considerations, the fact is that the cost of aerating a reactor effectively and efficiently requires a large expenditure of energy; and filtration through a membrane device requires a relatively high inlet pressure and high velocity of flow of concentrate through the membrane device; this requirement of high mass flow under elevated pressure in turn dictates high pump pressures, and high flow rates at elevated pressures which results in large pressure drops. All of which add up to such expensive operation that one skilled in the art would not expect that such a system might be economical with any combination of aerated bioreactor and membrane filtration unit.
In particular, the high energy requirements for pumping a suspension of organic solids from a bioreactor through a membrane filtration unit, and using the energy of the concentrate stream from the unit to entrain oxygen from an eductor requires that the kinetic energy of the concentrate stream be used to draw in and disperse the required oxygen-containing gas stream. Such a configuration has been suggested in French application 2,430,451 to Lambert et al filed Jul. 4, 1978. The efficiency of the system is adversely affected because dissipation of the kinetic energy of the recirculating stream provides no positive energy contribution to the recirculating stream.
The high mass flow and kinetic energy of the recirculating stream in the '451 reference contributes so much energy to the system that efficient mixing in the reactor results simply because of the high contribution of fluid energy, minimal residence time, and without concern as to the establishment of a recirculating pattern. Further, since a characteristic of an eductor is that its flow is limited by the mass flow of the recirculating stream and the resulting pressure drop generated in the eductor, under optimum conditions, one can typically only entrain less than about 1 volume of oxygen per 5 volumes of recirculating liquid, or, 1 volume of air per volume of recirculating liquid.
This physical limitation will be more readily understood by reference to the illustrative examples herebelow, in which a 30 liter reactor is provided with a recirculation stream of 6500 liter/hr (6.5 m.sup.3 /hr) so that the residence time is 16 sec. Of this stream, 3500 liter/hr goes to a single eductor which entrains 500 liter/hr of air. The inlet pressure of the recirculating stream into the eductor is 200 kPa gauge (30 psig). Though the membrane bioreactor system operates at low to medium pressure, in the range from about 100 kPa to 500 kPa, depending upon whether the membrane filtration device uses a microfiltration or ultrafiltraton membrane, a high mass flow of solids-containing concentrate is available for a recycle stream. This mass flow is high enough (i) to provide enough liquid as is required per unit of air entrained, (ii) to provide sufficient mixing to ensure homogenization of the biomass, and (iii) to establish a preselected recirculation pattern in the bioreactor.
I have found that the high cost of operation of the combination of a bioreactor and a membrane filtration device can be off-set with a particular form of in-line aerator positioned so as to provide a directed recirculating jet (referred to as a "tail-jet") within the reactor.
In particular, operation of a membrane filtration device requires accepting the possibility of serious membrane flux decline, that is the rate per unit area of membrane surface through which permeate leaves. Such decline is typically due to insufficient oxygen being introduced to meet the respiration rate of the biomass, resulting in clogging of the membrane's pores. This problem of clogging suggested that the use of a microporous gas diffuser means (such as a porous metal annular element) was contraindicated because of the proclivity of a microporous element to be clogged by biomass.
The challenge to provide the proper amount of air to an aerobic reactor has been addressed in numerous references such as Wastewater Engineering pp 492-502, Metcalf & Eddy Inc. McGraw Hill 1979; Activated Sludge Process: Theory and Practice by J. Ganczarczyk, pp 133-153, Marcel Dekker 1983; Wastewater Treatment Plant Design pp 241-258, Water Pollution Control Federation, 1977; and a host of patent references.
Favored among devices for introducing air into an aerobic reactor are jet aerators, because of the high oxygen transfer they efficiently provide, but have restricted flexibility because of their design. Jet aerators are also referred to as ejectors, injectors, venturi nozzles, and eductors. Such devices introduce oxygen and water in a two-phase stream at a velocity high enough to provide requisite mixing within the reactor. The two-phase stream leaves the jet aerator in the form of a free jet (referred to herein as a "tail-jet"), which having penetrated a certain distance into the surrounding liquid, loses its energy and breaks up into clouds of bubbles. (See Sorption Characteristics of Slot Injectors and Their Dependency on the Coalescence Behaviour of the System, by M. Zlokarnik Chemical Engineering Science Vol 34, pp 1265-1271, 1979; and, Design Manual -- Fine Pore Aeration Systems U.S. Environmental Protection Agency, Office of Research and Development, Center of Environmental Research Information, Risk Reduction Engineering Laboratory, Cincinnati, Ohio 45268, Sep. 1989).
Though much of the requisite oxygen transfer takes place in the jet aerator before the tail-jet is ejected into the reaction mass, the oxygen in the two-phase stream must also be transferred to the biomass in the reactor, and this requires a substantial residence time. Since the stream must also provide the motive force for adequate recirculation of the biomass, it is economical to provide such energy with only as high a recirculation rate of liquid as will provide the necessary oxygen requirement, since pumping costs provide efficient movement of liquid.
Prior art devices relied upon the recirculation stream to provide the kinetic energy for entrainment of oxygen and mixing of the reaction mass. There is little motivation to provide recirculation energy in a recycle loop by using the energy of air (oxygen and/or ozone) under pressure which air is required to feed oxygen to the biomass.
Yet I have done provided such energy derived from the air used. I have been able to do so because of the use of an in-line porous element having through-pores which place its interior and exterior surfaces in open fluid communication, referred to as a "gas micronizer" (or "micronizer" for brevity), so located as to provide a tail-jet to establish a recirculation pattern within the reactor. The micronizer is preferably located outside the reactor, and operated in the recycle loop in combination with the reactor and membrane device of my membrane bioreactor system, as will be described in greater detail hereinafter.